Plasma Physics Reports最新文献

Collective electromagnetic modes in weakly ionized plasma formed by multiphoton ionization of inert gas atoms, in which the Ramsauer–Townsend effect takes place, are studied. It is shown that at a relatively low energy of photoelectrons of the order of 1 eV, typical for multiphoton ionization, amplification of electromagnetic waves is possible. Amplification is possible both in the case of rare collisions of photoelectrons with neutral atoms and for collision frequencies higher than electron plasma frequency. At photoelectron energies somewhat higher than 1 eV, aperiodic instability can develop with growth rate whose value is comparable to electron plasma frequency. Detailed analytical and numerical analysis of the effect of collisions of photoelectrons with neutral atoms on the dispersion law of electromagnetic wave and the growth rates of instabilities is presented.

We studied an electric breakdown in an 80-cm-long discharge tube with the inner diameter of 1.5 cm (the so-called “long discharge tube”) in xenon at 1 Torr. The discharge was initiated by positive pulses of ramp voltage with a rise rate dU/dt on the order of 10^–1–10⁵ kV/s. The breakdown voltage was measured in darkness and upon tube illumination by fluorescent lamps, LEDs, or a diode laser. The effect of illumination depends on the slope of the pulse leading edge. The voltage drops at dU/dt > 100 kV/s, while growing at dU/dt < 100 kV/s. The voltage increases by a factor of six for the voltage slope of 0.1–1 kV/s. The dependence of the observed effect on radiation intensity, wavelength, and position of the illuminated area on the tube surface is studied. The pre-breakdown ionization wave behaves unusually under the described conditions: its speed and intensity of emission at its front grow in the course of wave propagation. Photodesorption of electrons from the tube surface as a result of which the wall near the anode becomes positively charged is assumed to represent the mechanism of the observed phenomena. This causes an increase in the breakdown voltage and accelerated propagation of the ionization wave. Additional experiments confirm the presence of the wall charge in the near-anode region under the discussed conditions.

The KINX and VENUS codes were used for simulation of the baseline inductive and steady-state scenarios of the ITER tokamak operation. The perturbations of plasma electron density and magnetic field caused by the Alfvén modes were calculated in the flux coordinates for these scenarios. The perturbation fields obtained were converted into the engineering coordinates in order to calculate the propagation of probe electromagnetic radiation of the reflectometer using the two-dimensional full-wave TAMIC RτX code in the expected geometry of the experiment. The calculations performed show that for the baseline inductive scenario, in the case of reflection of the extraordinary wave at the lower cutoff frequency from the high magnetic field side, the electric field relative perturbations of the reflected reflectometer signal correspond to the margin of linear range of the diagnostics operation or even go out of this range. It was found that in a number of scenarios, not only the electron density perturbations, but also the magnetic field perturbations significantly contribute to the total signal perturbations that makes even more difficult the further data interpretation. Another possible problem is the narrow frequency range of probing frequencies where the Alfvén mode can be observed. In addition to simulating the reflection of electromagnetic waves from plasma, it was analyzed also the possibility of measuring the Alfvén modes parameters when the extraordinary wave pass through the plasma in the transparency window between the upper and lower cutoff frequencies of the extraordinary wave (refractometry). It is shown that at the fundamental frequency, the phase perturbations range from 3 to 60 degrees, which makes it impossible to use the amplitude-modulated refractometer for analyzing signals. The “synthetic diagnostics” approach was used, which showed itself well for simulating the operation of reflectometers at plasma facilities.

The effect of random forces, which are caused by fluctuations in a complex plasma, on the dynamics of charged dust grains is studied. Analytical dependences are obtained for the kinetic energy, the autocorrelation functions of velocities, the mass transfer functions, and the mean-square average displacements of the grains in the case of a grain moving under the effect of two random forces. A method is proposed for accounting for more than two random forces of different nature. The possibility is discussed of simulating the motion of dust grains in complex plasma by the Langevin equations with a temperature that is not equal to the temperature of the surrounding gas.

This work is devoted to the project of a new-generation open trap, gas-dynamic multiple-mirror trap (GDMT), proposed at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. The aim of the project is to substantiate the possibility of using open traps as thermonuclear systems: a source of neutrons and, in the future, a thermonuclear reactor. The main objectives of the project are to develop technologies for long-term plasma maintenance in an open trap, optimize neutron source parameters based on the gas-dynamic trap, and demonstrate methods for improving plasma confinement. The magnetic vacuum system of the facility consists of a central trap, multiple-mirror sections that improve the longitudinal plasma confinement, and expanders designed to accommodate plasma flux absorbers. The facility is to be built in several stages. The starting configuration is broadly similar to the GDT facility and includes a central trap with strong magnetic mirrors and expanders. It solves two main problems: optimization of the parameters of the neutron source based on the gas-dynamic trap and study of the physics of the transition to the configuration of a diamagnetic trap with a high relative pressure β ≈ 1, which significantly increases the efficiency of the system. This work describes the technical design of the starting configuration of the facility and outlines the physical principles on which the GDMT project is based.

A high-voltage repetitively pulsed surface spark discharge propagating along the water–gas interface, when Ar is used as the gaseous medium, is studied. In the experiments, a generator with a storage capacitor energy of up to 1.6 J, a voltage of up to 20 kV, and a pulse duration of 2–3 μs is used. The energy characteristics of the discharge are measured as a function of its length from 40 to 140 mm. The UV radiation intensity is measured by actinometry in the wavelength range from 200 to 380 nm. It is established that the UV radiation yield along the discharge length is constant, almost independent of its length, and is directly proportional to the energy input into the discharge. The energy cost of a radiation photon is 150 eV. Quantitative estimates of the production of hydroxyl radicals depending on the length of the plasma channel and the energy input into the discharge are carried out.

Electron capture into the mode of synchronous gyromagnetic autoresonance in a combined mirror-type magnetic trap with a high-frequency field in a cylindrical resonator is studied. The averaged equations of motion of electrons in such a trap are obtained taking into account the terms of the first order of smallness in the high-frequency field amplitude. An equation for the resonant phase, which has the form of an equation for a nonlinear oscillator with a constant force, is derived to change the magnetic field with time according to a linear law in a weakly relativistic approximation. Based on the analysis of its solutions, a general criterion for the electron capture into the gyromagnetic autoresonance mode is obtained. Using the Bogolyubov method, the change in the energy of particles is studied, taking into account the time dependence of the parameters of the combined trap. It is shown that during autoresonance, the change in the electron energy with time occurs synchronously with the change in the magnetic field.

The structure of a low-pressure microwave discharge sustained by a standing surface electromagnetic wave (SEW) in a quartz tube filled with argon was studied. The standing wave was formed using a set of two flat metal mirrors, which formed an open SEW resonator. The plasma density profile and structure of the electromagnetic field of the SEW were studied in the pressure range from 0.25 to 10 Torr. The excitation of the standing wave allowed us to independently study the longitudinal E_z and transverse E_r components of the SEW electric field vector. It was confirmed experimentally that the oscillation phases of the components of the SEW are shifted by π. The excitation of the standing wave in the plasma column leads to the formation of local minimums and maximums of plasma density, whose period equals half the wavelength of the surface wave. At the same time, the spatial period of density modulation is close to the distribution of the E_z component of the standing SEW. It was shown that the formation time of the modulated structure of plasma density is close to the characteristic time of diffusion, while the degree of modulation increases with increasing pressure. It was shown experimentally that it is possible to produce a plasma column with plasma density modulation n_emax/n_emin ≈ 5 and a length of about 10 wavelengths.

The electron energy distribution function (EEDF) and the spatial profile of the electron density in the cathode–anode gap in a helium discharge are calculated within a one-dimensional model by the Monte Carlo method. Numerical studies are performed for experimental conditions known from the literature in a discharge with a hollow cathode: the cathode–anode distance of 3 cm, the helium pressure of 0.75 Torr, and the electric field strength in the discharge gap of 1.3 V/cm. The calculations are performed without and with allowance for the anode potential drop and the effect of electron reflection from the anode. The dependence of the form of EEDF on the energy spectrum of the electron source used in the calculations is also studied. In all variants of calculations, the main feature of the EEDF is retained, that is, a significant depletion of the low-energy part of the distribution function due to the effect of electron absorption by the anode. The calculated EEDF and the spatial profile of the electron density are compared with the available experimental data.

Cylindrical three dimensional dust–ion–acoustic (DIA) solitary waves (SWs) in a complex plasma medium consisting of nonthermal electrons, adiabatically warm ions, and immobile positively charged dust (PCD) species are studied. The reductive perturbation method, which is valid for small but finite amplitude waves, is used to derive the (3 + 1)-dimensional cylindrical Kadomstev–Petviashvili (cKP) equation (also known as cylindrical Korteweg–de Vries equation). The parametric regimes for the existence of solitary structures are shown. The plasma model under consideration supports both the positive and negative DIA SWs. Moreover, the effects of the physical plasma parameters (the ratio of the dust to ion number density, the nonthermal parameter, etc.) on the basic features (amplitude, width, and speed, etc.) of DIA SWs are discussed. Depending on the plasma parameters (the PCD and ion number density ratio, nonthermality of electron, and temperature ratio of ion and electron) the solitary pulses change their polarity. The present investigation may be helpful to the understanding of the properties of the DIA SWs in different astrophysical plasma environments as well as in laboratory devices.